"We were quite intrigued by the cable structures across the creek—which to our knowledge had never been done before—but the project was a complex challenge," says George T. Vavrek, managing partner for construction manager-constructor LNJV, a joint venture of Linbeck Group LP, Houston, and Nabholz Construction Services, Conway, Ark.
At times during construction of the seven-acre Crystal Bridges Museum of American Art in Bentonville (population 37,000), team members must have felt they were up a stream without a paddle, due to new challenges. Incompetent limestone, which prompted a redesign from footings to micropiles, halted foundation work for seven months in 2007. And in 2009, when the Nov. 11, 2011 opening seemed in jeopardy, the owner realized it needed help and organized an intervention to get the job back on track.
Thanks to the owner's move, a regrouping and lots of preconstruction engineering, the saga of the museum in a ravine is ending well. "We are completing on time—incredible as that sounds," says Vavrek.
The design for the campus by Moshe Safdie and Associates Inc., Boston, consists of one-, two- and three-story buildings with such distinct shapes they have nicknames. The two cable-roof buildings, each perched on a weir that will dam Crystal Spring to form ponds, are called "armadillos." The third cable-roof building is called "the horseshoe crab." One gallery was dubbed "the football," another "the elbow." In plan, the 10 linked buildings form the shape of the number nine. Six will contain art, for a total of 40,000 sq ft of gallery space (see p. 32).
Like Hammocks
The cable roofs, framed by glue-laminated wood arches, are the trickiest of all the structures. Though cables are very efficient in tension, they stretch and deflect like hammock ropes, making the roofs and all their layers especially challenging to erect. "It was a real engineering feat to be able to suspend cables and set the beams," says Jimmy Kepple, vice president of operations for Bosworth Steel Erectors Inc., Dallas, which is erecting the structures.
Even without the roofs, the job would have been far from conventional. "It's a very challenging job from a civil-engineering standpoint," says Brent Massey, principal of the local civil engineer CEI Engineering Associates Inc. Buildings aren't typically built where there is so much potential for flooding, he adds.
Safdie's architectural concrete walls, with varying radii, and toroidal rooflines, which curve in two directions, have also been taxing. "It was not obvious that people in rural Arkansas knew how to do architectural concrete," says Cristobal Correa, associate principal for structure in the New York City office of the job's consulting engineer, Buro Happold Inc.
Logistical hurdles came with the territory. The terrain is steep and rough, with a 120 ft drop in elevation from the construction trailers and concrete batch plant to the site a half-mile away. There is little room to maneuver in the diverted streambed, blasted out to be 550 ft by 800 ft. For example, with no place to turn around, the flatbed that delivers the glulams has to be unhitched, picked up and turned by the tower crane.
The charge from Walton, who chairs the non-profit museum's sole funder, the Walton Family Foundation, was to build the complex but tread lightly on the landscape. LNJV did its best. For instance, it set the height of the tower crane 280 ft above the pond bottom so that the boom would clear the tree line.
Crews began blasting limestone, excavating and diverting the creek in late 2006. It didn't take long for the first hiccup. "The rock fissured in 2007 as we started excavating," says Vavrek.
Work started again seven months later, after engineers replaced footings with micropiles. LNJV's strategy was to tackle the most difficult buildings first, working north to south. But by mid-2009, with above-grade concrete work well under way, museum officials realized the Nov. 11, 2011, public opening was in jeopardy. "The scale was large, and we needed professional expertise on-site," says Rod Bigelow, the museum's deputy director of operations and administration.
That October, the museum brought in Hines as an owner's representative to evaluate and restructure the project and help manage communication between the design and construction teams. "It was a daunting task," says Lawrence Peszek, vice president of construction in the Minneapolis office of Hines.
